How to select all objects from different Collections or Hierarchies?
I think this is the method so far if there is not many Collections or Hierarchies to select.
Here are the steps:

Turn on &quot;Extend Mode&quot; for select grouped, shortcut F9


2.Select one Parent, Shift+G, select children.

3.Hold Shift, select another Parent, Shift+G for Children selection...
so on..

If there are 50 Hierarchies....
But I think it is not hard to make a Python script for this kind of selection.

Select all parents.
Run Python, Loop for all these selected parents, Each loop for children selection extended.

Hope easier life for using outliner...